A modern Fourier Transform Infrared FT-IR spectrophotometer consists of two basic parts: (1) an optical system which includes an interferometer through which an infrared light beam is directed before passing the beam through a sample and (2) a dedicated computer which is used to control the spectrophotometer and analyze spectral information contained in the light beam produced. The advantages and improved performance of a FT-IR spectrophotometer result from the use of an interferometer, rather than a grating or prism to obtain variance in the intensity as a function of wavelength in the infrared beam applied to the sample in generating spectral data indicating the sample com position. An interferometer permits measurement of the entire spectral profile of a sample in a fraction of the time previously required while increasing the amount of information obtained.
The operation of a Michelson interferometer as applied to FT-IR spectrophotometry is well known. The Michelson interferometer consists of a pair of perpendicularly arranged optical paths, each having a reflector or mirror positioned at its end to reflect light traversing the path. One mirror is fixed to define a first fixed length light path. The other mirror is movable to increase or decrease the length of the second light path. A light beam entering the interferometer is optically split into two beam portions by a beam splitter so that an individual portion of the beam will traverse each of the optical paths. After reflection from their respective mirrors the beam portions are recombined through the beam splitter to constructively and destructively interfere causing interference phenomenon in this reconstructed light beam. This produces an intensity modulation of the light beam. The reconstructed light beam is thereafter directed through a sample and focused onto a photodetector for measurement of intensity and intensity variance of the light wavelengths comprising the infrared light beam.
The intensity variation of the light wavelengths comprising the reconstructed light beam depends in part on the difference in length of the optical paths over which the portions of the beam travel. Generally, when the movable mirror is scanned at a constant velocity, the intensity of the reconstructed light beam will modulate in a regular sinusoidal manner for any selected wavelength of light within the infrared light wavelength ranged passing through the interferometer.
A typical infrared light beam emerging from the interferometer is a complex mixture of frequencies (light wavelengths) due to the polychromatic nature of the entering infrared light. After the infrared light beam has passed through a sample material, it can be photometrically detected to determine wavelengths of light which have been absorbed by the sample. This is accomplished by measuring change in the regular sinusoidal pattern of intensity variation expected in the light beam exiting the interferometer. Measurement of the differences in the characteristic sinusoidal intensity pattern indicates those wavelengths of light which are absorbed by the sample through Fourier Transform analysis. Infrared light absorbance characteristics provide spectral data from which the matter comprising the sample can be determined.
The output signal of a detector detecting the intensity modulation of the emerging light beam can be recorded at very precise intervals during mirror scan of the interferometer, to produce a graphical plot known as an interferogram. The interferogram is a record of the output signal produced by the light detector (beam intensity) as a function of the differing length optical paths traversed by the infrared beam in the interferometer. Successive interferograms of the sample are obtained and co-added to obtain an average interferogram having improved signal-to-noise characteristics to aid sample analysis. The averaged interferogram provides information and data relating to the spectral characteristics of the sample material. After mathematical preparation, a Fourier transform calculation is performed on the interferogram data to obtain a spectral fingerprint of the sample composition. The results are compared against known reference data to determine the composition of the sample.
Most Fourier transform techniques require averaging of a large number of interferograms in order to obtain accurate results. As many as 50 interferograms may be averaged. It is important for interferograms to be precisely reproducible for their accurate averaging. Since an interferogram is created as a function of the movable mirror position, more accuracy in the interferogram (and in the resultant Fourier transformation) will be obtained if more accuracy is obtained in the determination of mirror position when the data are measured which define the interferogram.
Since the movable mirror in the interferometer is scanned while measurements of the light beam are taken, to accomplish accuracy in and reproducibility of an interferogram, both the rate of data measurement and mirror velocity must be very precisely controlled. Alternatively, the relative position of the mirror may be measured when a data measurement is taken. Modern systems accomplish sampling rate and mirror velocity control ( and/or mirror position measurement) by passing a laser beam concurrently through the interferometer with the infrared light. The laser beam is used to directly measure the movement and/or position of the movable mirror. Since the laser beam traverses the same change in optical path as the infrared light beam, the laser beam provides a detectable monochromatic wavelength with an interference pattern containing information of the scan velocity of the movable mirror. The interference pattern also serves to indicate the position of the mirror during a scan, and to initiate and correlate the collection of data points at uniform and equal intervals of mirror displacement.
In a conventional system, when the movable mirror is moving at a constant velocity, a Doppler shift in frequency is generated in the portion of the laser beam traversing the second light path which is changing in length. When the laser beam portion exhibiting a Doppler shift in frequency is recombined with the laser beam portion traversing the first fixed length light path, a modulated frequency laser beam exhibiting a measurable intensity modulation having a distinct lower frequency (beat frequency) is produced, yielding varying intensity or fringe patterns which may be analyzed to determine mirror position and/or velocity. The frequency of the beat is useful because the frequency of the laser beam is much too high for measurement by common detectors.
Conventional systems generally drive the moving mirror of the second light path at a velocity which produces a 5 KHz intensity modulation or beat frequency in the exiting laser beam. This beat frequency is equal to the magnitude of the Doppler shift in frequency because it equals the difference in frequency between the recombined light beam portions. At increased mirror scan velocities, the beat frequency will increase providing increased resolution while at slower mirror scan velocities the modulation frequency will decrease. Precision with this technique can be maintained to approximately one cycle in 5,000 to provide very accurate position and velocity information.
In a conventional system, however, a movable mirror must be in motion to obtain a Doppler shift in frequency in the light beam traversing the second light path, and thus a measurable beat frequency. To explain, when the movable mirror is stationary, the portions of the laser beam traveling along their individual light paths of the interferometer are recombined to constructively or destructively interfere, but form an identical frequency laser beam without modulation since no Doppler shift in frequency has been introduced in either portion of the laser beam. The emerging recombined beam exhibits no intensity modulation and no beat. Thus when the mirror is not moving, there is no information obtainable from the recombined laser beam which can be used to determine mirror position or mirror velocity. In operation this occurs at every instance that the movable mirror reaches the end of its scan and stops to proceed in the other direction.
Furthermore, in a conventional interferometer system the Doppler shift in frequency generated by a scanning mirror produces the same intensity modulation effect in the recombined laser beam independent of the direction of mirror travel. For instance, a 5 KHz beat frequency may be obtained for travel of the mirror in either a forward or reverse direction. Thus, it is impossible to determine the direction of mirror travel from the emerging recombined laser beam, even though the difference in length of the optical paths may be increasing or decreasing. This shortcoming generally requires additional circuitry to obtain an indication of the direction of mirror travel so that the exact position of the mirror may be determined at any given time.
Lastly, with conventional systems modulation in the recombined laser beam becomes very difficult to measure as the scan velocity of the mirror becomes very slow. For instance, for a 0.3 centimeter per second scan velocity, a beat frequency of 5 KHz is obtained in the recombined laser beam. However, if the mirror is driven at a scan velocity of 0.03 centimeters per second, the beat frequency is reduced to 0.5 KHz. Thus, as the scan velocity is decreased, the beat frequency in the recombined laser beam is decreased to a level which is difficult to measure with modern electronic detectors, providing less accuracy in and resolution of mirror control.
An FT-IR spectrophotometer has limited resolution in sample identification determined by its ability to produce and reproduce, an interferogram. The only moving part fundamental to the optical system is the movable mirror of the interferometer. This part largely determines the accuracy with which a spectrophotometer can generate interferograms. The accuracy with which the spectrophotometer can analyze a sample is directly related to the accuracy and reproducibility of the interferogram and thus the ability of the instrument to control and determine the velocity and position of the movable mirror.
The conventional use of a laser reference to control and determine the velocity and position of the movable mirror continues to suffer limited precision and control ability. Improvements in the precision with which mirror position can be measured and mirror velocity controlled will necessarily produce significant improvement in the accuracy with which an FT-IR spectrophotometer can analyze a sample substance.